RC4Magic-900 and RC5 EASS systems use Frequency Hopping Spread Spectrum to wirelessly transmit data.  Each small packet in a data stream is transmitted on a different radio channel.  The specific channels that are used can be selected by the RC4 user with the channel mask settings.

RC4Magic-900 has 63 channels available, of which 25 are required to be used to meet FCC regulations.  RC5 EASS has 101 channels available, of which 50 must be used.  For each of these systems, a different default channel mask is usually used with each different system ID (ID0, ID1, ID2, and ID3).  For additional data security, a custom channel mask can be created.  An Excel spreadsheet is available to assist with creating custom channel masks for RC4 systems that use them.